The present invention is in the field of solid-state switching devices and, more particularly, packaging and thermal management of solid-state switches.
In some applications, a solid-state switch (e.g., an insulated gate bipolar transistor (IGBT) or a metal-oxide field-effect transistor (MOSFET)) may perform high frequency switching at high power levels. As a consequence, the switch may produce heat. Multiple switches may be assembled in series or in parallel combinations in order to service some high voltage and/or high power applications such as motor control systems. In this context it may be necessary to provide cooling systems for the switches.
Various cooling and heat-sinking systems for switches have been employed in the prior art. A typical prior-art cooling system may employ electrically non-conductive heat transfer elements in contact with electrodes of a semiconductor die or switch (e.g. a MOSFET or IGBT). The heat transfer elements may convey heat into a solid or fluid-filled heat sink. See for example, U.S. Patent Application 2004/0207968. In such prior-art arrangements, heat generated within the die must be conducted through the electrically non-conductive heat transfer element before the heat may be removed from the switch. Consequently, undesirable thermal gradients may develop within the die.
Some prior-art high-power solid-state devices or switches are assembled by soldering multiple semiconductor dice to a substrate and then using wire bonds to electrically connect emitters, collectors and gates of the dice together in parallel/series arrangement to obtain a device with higher current or higher voltage blocking capability. The substrate may be electrically isolated and placed on a heatsink. In high power/high-frequency applications, connection points for these wire-bonds may be a source of undesirable stray inductance, voltage drop, power dissipation and variation in propagation delays (to power and/or control signals) due to presence of different paths with varying lengths dictated by layout geometry.
In such prior art assemblies, reliability may be compromised because of disconnection of bond wires due to corrosion of wires, bond lift-offs, heel cracks, and reconstruction of Al-metallization on the chips are identified as significant limiting factors for device reliability (see for example, “Selected failure mechanisms of modern power modules” by Mauro Ciappa, Microelectronics Reliability 42 (2002) 653-667).
Additionally, in these prior-art assemblies only one-side is typically available and used for cooling of the two-sided dice. This is because one of the two sides is used for internal wire-bond connection of dice. In this regard, an equivalent thermal-resistance from junction to base-plate may be undesirably large due to the existence of a stack-up of multiple-layers of different materials including but not limited to base-plate, direct copper bonded ceramic substrate, the semiconductor die itself, metallization layers and various attachment materials such as solder and/or glue). This large equivalent thermal resistance may dictate de-rating of the device current significantly and higher thermal management may also be required which in turn may drive-up the overall cost, weight, volume of a power conditioning system in which the devices may be employed.
Presence of high thermal gradients within prior-art packaged solid-state switches may diminish reliability of the switches. One particular failure mechanism may occur when the switches are employed in applications with varying atmospheric pressures such as in airborne applications. In these cases, moisture may enter a switch package at entrance points for connection pins (power and/or gating/PHM). Various attempts have been made to provide sealed packages with special sealing around the connection pins to prevent moisture intrusion resulting from altitude changes. So-called “press pack” assemblies may be examples of such prior-art devices. These prior-art sealed packages are expensive and difficult to produce.
As can be seen, there is a need to provide cost-effective and easy-to-manufacture thermal management and packaging systems to overcome these problems. In particular, there is a need to provide such switches in a configuration that may be effectively cooled. Additionally there is a need to provide such switches in a package that may reliably prevent moisture intrusion.